Disadvantages of importance sampling
If a lamp is too dim to add any significant lighting to other objects (except for very close ones), including it in MIS just wastes computations for no real purpose. If you are using branched path tracing, it will sample all lights on every sample (with MIS). So you want to make sure it is only spending time doing samples that might return something other than 0 (or close to it). If you are using the simple progressive path tracing mode, it only samples one lamp per sample. So if a lamp won't contribute much, you don't want it to use MIS because it might steal the sample from a more useful lamp.

tl;dr: disabling MIS is a way to tell Cycles "this lamp is too dim to illuminate much of anything, don't waste your time trying".